This invention relates to improvements in an exhaust gas purifying system for purifying exhaust gas or burnt gas discharged from a combustion device such as an internal combustion engine, a burner, a furnace or the like, and particularly to the improvements in the exhaust gas purifying system using a catalyst for reducing nitrogen oxides (NOx) in exhaust gas discharged under a lean-burn operation in which an air-fuel mixture leaner than a stoichiometric level is supplied the combustion device.
It is well known that a three-way catalyst was used to oxidize carbon monoxide (CO) and hydrocarbons (HC) and to reduce nitrogen oxides (NOx) contained in exhaust gas discharged from an internal combustion engine of an automotive vehicle or the like. The three-way catalyst can effectively function in a stoichiometric atmosphere (exhaust gas having a stoichiometric air-fuel ratio). However, such three-way catalyst cannot function to reduce nitrogen oxides in a lean atmosphere (exhaust gas having a leaner air-fuel ratio than the stoichiometric level). In this regard, a technique for reducing nitrogen oxides in the lean atmosphere has been made in Japanese Patent No. 2600492, which a NOx trap agent can trap NOx in the lean atmosphere and can release NOx by enriching air-fuel ratio of exhaust gas flowing to the NOx trap agent so as to reduce the released NOx.